February 28 & March 1, 2005
Topic: Early development of chick and mammal
Assigned reading:
Ch 11 pp354-384
Minireview on studies by Gardner & Zernicka-Goetz
Chick and mammalian eggs are dramatically different. Chicken eggs have a large
yolk mass that is not penetrated by cleavage planes. Mammalian eggs, in
contrast, have little yolk, and cleavage is holoblastic. However, by the
gastrulation stage, chick and mammalian embryos look very similar.
EARLY DEVELOPMENT IN BIRDS (chicken)
CHICK CLEAVAGE
Chick cleavage is “discoidal”:
The cleavage planes are meroblastic: they do
not proceed through the entire egg. The young embryo is formed in a small disc
of cytoplasm at the animal pole. (see Figure 11.12)
The early cleavage cells are continuous with each other, with the yolk at their bases
(Fig. 11.12) In later cleavage stage embryogenesis, the "blastoderm" is 5 -6 cell
layers thick.
Figure 11.13 &11.14A
In the center of the embryo, the deep cell layers die and are shed, leaving a disc one
cell deep. The edges are still several layers deep. The center of the embryo
appears relatively clear and is called the "area pellucida". The edge appears
more dense and is called the "area opaca". At the interface between these two
cell populations is the marginal zone.
Turn the disc on its side (Fig 11.13)
The space between the embryo and the yolk is called the subgerminal cavity (don’t
confuse with blastocoel).
Formation of the blastocoel:
The "area pellucida" is one cell layer thick. That cell layer is called the epiblast.
To form a blastocoel, the embryo must create a second layer, called the hypoblast.
The cells that form the hypoblast come from two sources:
Cells delaminate from the epiblast.
Migration of deep cells from the posterior region.
The epiblast and hypoblast are joined at the edges, and the space between them is
the blastocoel.
The chick embryo will be derived entirely from the epiblast.
The cells of the hypoblast will contribute to extra-embryonic tissues.
D/V AXIS FORMATION
The dorsal-ventral axis is determined by the pH gradient between the albumin (egg
white) above the disc (pH 9.5) and the subgerminal space (pH 6.5). This
differential is created by active transport of ions and water across the cell layer
as the subgerminal space is created. There is also a membrane potential
difference, such that there is a higher concentration of positive ions in the
subgerminal space than in the albumin.
Albumin
basic, negative
Subgerminal space
acidic, positive
The dorsal side of the embryo is the side contacting the albumin, and ventral side is
adjacent to the yolk.
CHICK GASTRULATION
Gastrulation initiates with the formation of the "primitive streak" or "primitive groove".
The primitive streak is analogous to the amphibian blastopore.
The primitive streak is first evident as a thickening of the epiblast at the posterior
region of the embryo. See Figure 11.14.
The visible "streak" is due to the ingression of endodermal precursors from the
epiblast into the blastocoel. The top of the epiblast defines the dorsal side of the
embryo.
NOTE:
Gastrulation in chick involves the movement of individual cells.
Contrast with frogs, in which gastrulation involves movement of sheets of cells.
Ingression of individual cells requires that cells lose affinity for their neighbors and
initiate migration.
The streak eventually extends 60 - 75% of the length of the area pellucida.
The endodermal and mesodermal precursors form two populations. The deep cells
displace the hypoblast anteriorly. These cells will contribute to endodermal
structures. The mesodermal cells spread between the endoderm and the
epiblast, and they will form the mesodermal structures. Figure 11.15
Regression of the primitive streak.
As gastrulation completes in the anterior, further development proceeds, and the
primitive streak moves posteriorly (back towards the blastopore). In avian (and
mammalian) embryos, there is a gradient of developmental maturity, in which the
anterior is more developmentally mature than the posterior. This means that the
anterior initiates organogenesis before the posterior completes gastrulation.
The ectoderm expands via epiboly to surround the entire yolk mass. Thus, the yolk
becomes enclosed within the embryonic tissue. Cell division plays an important
role in the expansion of the ectoderm.
Henson's node is the organizer.
The anterior end of the primitive streak (or primitive groove) is called Henson's node.
Henson's node is functionally similar to the dorsal lip of the amphibian
blastopore. The cells of Henson's node secrete inhibitors of TGF-beta ligands.
The chick equivalent of the N.C. and the organizer
(Koller’s sickle and Henson’s node)
Experimentally, how would you identify the chick organizer? (See Figure 11.20)
How would you identify the chick equivalent of the N.C.?
The A-P axis of the chick is thought to be dictated by gravity as the embryo roles
down the oviduct. The blastoderm is less dense than yolk and stays on top the
egg. The more elevated region of the blastoderm become the posterior. See
Figure 11.17
Koller’s sickle, which overlaps with the posterior marginal zone (Fig 11.19A) is
equivalent to the amphibian Nieuwkoop Center, as it can induce gastrulation in
neighboring tissue.
Study question:
Assume that you are a researcher who is trying to identify the molecules that mediate
organizer activity in Henson’s node. You have several candidates that you would
like to test. Describe an experiment that would allow you to test the hypothesis
that a given molecule plays an important role in the function of Henson’s node.
1. correlative; is it expressed at the appropriate time & place.
2. loss-of-function: is it required?
3. gain-of-function: is it sufficient?
Note: Any one experiment can support or refute the hypothesis, but multiple
experiments are needed to build a strong case for a given hypothesis or model.